This invention relates to a process for preparing sugar substitute xylitol from xylan containing natural products by acid hydrolysis and catalytic hydrogenation.
Xylitol is increasingly being used as a sugar substitute for diabetics and as a component of intravenous feeding solutions. Known processes for obtaining xylitol begin by producing xylose by acid decomposition from natural products containing xylan. The xylose contained in the acid decomposition solution is then freed of accompanying substances and isolated in pure form. The pure xylose is then hydrogenated under pressure to form xylitol, whereupon the xylitol is isolated from the hydrogenating solution and obtained in pure form.
These known processes, however have the disadvantage that two separate isolating processes are required, namely the intermediate isolation of xylose and final isolation of xylitol. It has hitherto been thought that hydrogenation of xylose is possible only when the xylose is in the isolated state because the accompanying substances produced in the acid decomposition have a harmful effect on the hydrogenation catalyst. Sometimes the acid decomposition produces acetic acid which reacts with the catalyst and soon renders it useless. This is especially critical in the case of Raney nickel catalysts, and even in the case of noble metal catalysts, such as ruthenium catalysts. The acetic acid in some cases can react with the support which also renders the catalyst useless. Nor does it suffice to neutralize the hydrolysis solution, because the hydrolysis of the acetate that forms is constantly establishing a pH of 5 at which the catalyst or support begins to dissolve.
The isolation of xylose brings about a separation of the glucose that also develops in the decomposition, so that the hydrogenation can produce xylitol only, rather than a xylitol-sorbitol mixture, for it is known that it is less difficult to isolate pure xylitol from an aqueous mixture of xylitol and sorbitol.